Originally Posted by
2001racer
... Any explanation necessary other than "I'm doing a mileage run..."
Some immigration officers will be fine with that much, if they ask at all. Others, who haven't encountered our particular form of insanity before, may want a longer explanation, so you might want to rehearse something that a lay person can understand.
I had to go through the long version at Perth a few years back. An hour later, having walked in the car park and bought a T-shirt, coming through in the other direction, the same immigration officer spotted me at the far end of the wide hall. He stood on a chair, waved a newspaper in the air, and shouted "Come on down here so you don't have to explain it to anyone else again!"
The typical English officer is a bit more restrained than the typical Aussie, of course. I doubt that will happen at LHR, aside from the odds at a much larger airport working against it. (Even PER has expanded since.)
Time-wise, piece of cake.